1. Field of the Invention
The present invention relates generally to a class of field effect transistors based on III-nitride materials, and relates more particularly to a field effect transistor that uses spontaneous polarization fields to provide enhanced conductivity while providing improved electrical insulation under the gate structure.
2. Description of Related Art
Development of devices based on III-nitride materials has generally been aimed at high power-high frequency applications such as emitters for cell phone base stations. The devices fabricated for these types of applications are based on general device structures that exhibit high electron mobility and are referred to variously as heterojunction field effect transistors (HFETs), high electron mobility transistors (HEMTs) or modulation doped field effect transistors (MODFETs). These types of devices are typically able to withstand high voltages such as in the range of 100 Volts, while operating at high frequencies, typically in the range of 2-100 GHz. These types of devices may be modified for a number of types of applications, but typically operate through the use of piezoelectric polarization fields to generate a two dimensional electron gas (2DEG) that allows transport of very high current densities with very low resistive losses. The 2DEG is formed at an interface of AlGaN and GaN materials in these conventional III-nitride HFETs. However, a drawback of these types of devices is the limited thickness that can be achieved in the strained AlGaN/GaN system. The difference in the lattice structures of these types of materials produces a strain that can result in dislocation of films grown to produce the different layers. This results in high levels of leakage through a barrier layer, for example. The addition of an insulator layer can reduce the leakage through the barrier, and typical layers used for this purpose are silicon oxide, silicon nitride, sapphire, or other insulators, disposed between the AlGaN and metal gate layers. This type of device is often referred to as a MISHFET and has some advantages over the traditional devices that do not have an insulator layer. However, there are several drawbacks to this type of design. First, the additional interface between the AlGaN layer and the insulator results in the production of interface trap states that slow the response of the device. Second, the additional thickness of the oxide, plus the additional interfaces between the two layers, results in the use of larger gate drive voltages to switch the device.
Conventional device designs using nitride material to obtain nominally off devices rely on this additional insulator to act as a confinement layer, and reduce or eliminate the top AlGaN layer. These devices, however, typically have lower current carrying capacity due to scattering at the GaN/insulator interface.
Accordingly, it would be desirable to produce an HFET switching device that has a low leakage characteristic with fewer interfaces and layers that can still produce high current densities with low resistive losses. Presently, planar devices have been fabricated with GaN and InAlGaN alloys through a number of techniques, including MOCVD (metal organic chemical vapor deposition) as well as molecular beam epitaxy (MBE) and hydride vapor phase epitaxy (HVPE).
Materials in the gallium nitride material system may include gallium nitride (GaN) and its alloys such as aluminum gallium nitride (AlGaN), indium gallium nitride (InGaN) and indium aluminum gallium nitride (InAlGaN). These materials are semiconductor compounds that have a relatively wide direct bandgap that permits highly energetic electronic transitions to occur. Gallium nitride materials have been formed on a number of different substrates including silicon carbide (SiC), saphire and silicon. Silicon substrates are readily available and relatively inexpensive, and silicon processing technology has been well developed. However, forming gallium nitride materials on silicon substrates to produce semiconductor devices presents challenges that arise from differences in the lattice constant, thermal expansion and bandgap between silicon and gallium nitride. Differences in the properties between gallium nitride materials and substrates can lead to difficulties in growing layers suitable for many applications. For example, GaN has a different thermal expansion coefficient, i.e., thermal expansion rate, than many substrate materials including saphire, SiC and silicon. The thermal expansion differences can lead to cracking of the GaN layer deposited on such substrates when the structure is cool, for example during processing. GaN also has a different lattice structure than most substrate materials. The difference in lattice constant may lead to the formation of defects in gallium nitride material layers deposited on substrates. Such defects can impair the performance of devices formed using the gallium nitride material layers.
The problems attendant with the lattice mismatch between GaN and traditional substrate materials are also prevalent in material layer structures involving GaN and GaN alloys. For example, GaN and AlGaN materials have lattice structures that differ significantly enough to produce interface strain between the layers. This strain contributes to the piezoelectric polarization that in turn produces the high levels of electron charge at the interface, resulting in high current carrying capacity. In many previous devices, the fields generated by the piezoelectric polarization are maximized through increasing the strain to improve the characteristics of the devices. However, increasing the strain by increasing the content of aluminum in the AlGaN/GaN layer structures causes the same detrimental effects as the strain and lattice mismatch associated with growth of GaN on the variety of substrates mentioned above, including defect generation and cracking. For example, a major drawback of the use of high strain AlGaN/GaN materials in tradition III-nitride HFETs to permit high current densities with low resistive losses is the limited thickness that can be achieved in constructing the strained AlGaN/GaN system. The strain results in dislocation generation during film growth and results in high levels of leakage through the barrier layer. While additional insulator layers can permit thicker strained AlGaN/GaN systems to be constructed, the confinement layer produced by the additional insulator results in higher threshold voltages and switching losses due to the higher voltage required to turn the devices off.
Accordingly, it would be desirable to produce a III-nitride material field effect device with greater current carrying capacity, while being operable to withstand high voltages and reduce or practically eliminate gate leakage.